Changes in the Colour of Cars

For five weeks in the late summer of 1985, Aretha Franklin’s ‘Freeway of Love’ topped the Hot R&B/Hip-Hop Songs chart, making it her highest charting single in twelve years.  Even then it hearkened back to times when cars and romance – glamour and fun all went together.

The romance is obvious: —“We goin’ ridin’ on the freeway of love, in my pink Cadillac,” —but no more so than even the earliest of car songs from the turn of the century.  In Aretha’s honour, at her funeral, some of her associates in the Mary Kay Cosmetics company—famously known for rewarding its top consultants with pink Cadillacs—were asked to accompany the funeral procession.

Some reports say about 130 pink Cadillacs lined Detroit’s Seven Mile Road, with one person in attendance quoted as saying “They’re coming from everywhere… from as far as Texas, Nebraska, Florida, North Carolina, Maryland.” So … just when it seemed cars are only about ‘green’ and other such utilitarian concerns these days, up pops this bit of kitschy-cool as a reminder that colour is still very much an essential in vehicle design and marketing.

However, this essential need for car colour variety presents a ‘21st century’ problem.  This is because radar and lidar, the two main sensor systems of autonomous vehicles, function best when the colour of the vehicle to which they are attached reflects the widest range of wavelengths, radio waves and infrared respectively.

Black cars, which comprise 17% of new cars sold around the world, actually absorb much of the signal transmitted from these sensors, thereby degrading their range.  White cars, comprising 37% of global new car sales are the ideal colour for sensor-bearing vehicles, but colour variety, as we have seen is still considered essential.

Reportedly, because of this car colour challenge, the chemistry of car paint and how it is applied, already a complex high-tech aspect of car manufacturing, is becoming even more so. PPG, a Pittsburgh-based supplier of industrial paints and coatings is developing paint that “at a molecular level… whatever colour… it appears to be to the human eye, will still be highly reflective to the signal from a car’s sensors.

In their efforts to make dark colours reflective, an April 2018 issue of the The Economist Magazine reports that PPG’s paint technologists have taken inspiration from the eggplant.  The temptation is to think vegetable eggplant but it turns out the combination of the dark-purple skin over the white interior flesh is an ingenious adaptation that allows this dark vegetable to remain cool in a sunny field.

Instead of absorbing infrared radiation, the dark-purple skin allows such wavelengths to pass directly through it where, upon meeting the white interior, they are reflected back out again.  PPG’s aerospace division has applied this ‘solution’ to keep aircraft painted with dark colours cool.  Applying it to sensor radiation is the approach currently under development.  Similarly, the reverse approach could also be used to “tone down, in relevant frequencies, (for example) road signs … designed to be super bright to people but thus risk blinding lidars.”